It's also getting really late and really warm for Brussels sprouts to be growing in Georgia. They're cool-weather veggies so you definitely need to give them some sort of soluble fertilizer that they can use right away if you hope to get a crop before the heat ruins the sprouts. How about fish emulsion?
Btw, I grow Brussels sprouts and other cabbage-y things in the winter here, although I'm a lot further south than you. I plant them around the beginning of October but you could probably plant in late August or any time in September. You'll get tastier, tender sprouts if you can get them maturing in cool weather.
